The present invention relates to radio proximity fuzes and more particularly to an active-passive radio proximity fuze employing a coincidence gate to insure activating the fuze only when it is sufficiently close to the target to destroy the target. Normally, an active passive proximity fuze employs three receiver channels. An active receiver channel senses the presence of signals reflected from the target when the target appears within the lethal volume surrounding the warhead of the missile and is the channel which generates the firing signal to detonate the warhead. The second of the three receiver channels is the guard channel and it employs a receiving antenna whose beam pattern is tilted more forward (in the direction of the missile line of flight) than the active channel antenna beam pattern. The guard channel can then sense the presence of a jammer equipped target some time before the target appears in the lethal volume. The beam pattern configuration is such that a pronounced decrease in gain occurs at the fuze-to-target angle when the target is about to appear in the lethal volume about the warhead.
The third receiver or passive channel, unlike the active channel, responds to a jamming signal in order to initiate detonation of the warhead. The passive channel receives signals from the same antenna that is employed with the active channel, and may share some other components with the latter, such as the mixer and intermediate frequency amplifier. Like all antennas, the antenna employed with these two channels has a number of minor lobes about its main beam. The minor lobes that are pointed in the direction of flight of the missile sense the jamming energy emanating from the target long before the target appears within the lethal volume. Because the passive channel responds to jamming signals, it must be maintained in a disabled state until the target appears within the lethal volume of the missile in order to prevent detonation of the warhead prematurely.
As the missile approaches the jammer equipped target, the signal within the guard channel is continually rising because of the closing distance. The guard channel is equipped with an automatic gain control (AGC) system in order to accommodate the wide amplitude range of signals anticipated. Eventually, the signal within the guard channel attains an amplitude sufficient to close a relay, which then completes the path between the passive channel detector and the firing circuit. The firing circuit is employed for the purpose of initiating warhead detonation by signals from the passive channel. The passive channel is now activated. Closure of the relay also results in the application of AGC potentials from the guard channel to the passive channel. The sensitivity of the passive channel is thereby reduced, preventing response to jamming signals sensed by the front minor lobes of the antenna. Preventing response to jamming signals at this time is necessary because detonation would be premature, as the missile and target are too far away from each other.
The guard channel antenna pattern configuration is such that when the missile arrives close to the target, the signal within the guard channel decreases, because of a decrease in antenna gain at the missile-to-target angle. The AGC potential applied to the passive channel also decreases, and the sensitivity of the channel rises.
When the lethal volume of the warhead envelops the target, the jamming signal emanating from the target enters the passive channel by way of the major lobes of the antenna and is applied to the firing circuit. Because the sensitivity of the passive channel had previously been increased, and because of the higher gain of the antenna major lobes, the signal at the firing circuit is sufficient to initiate detonation of the warhead and the target is destroyed. Thus, the successful operation of the passive portion of the fuze is dependent upon a decrease of guard antenna gain, due to the geometry of approach near the lethal volume, followed by an increase in active-passive antenna gain when the missile arrives at its destination.
The prior known systems have the inherent disadvantages of the passive channel becoming activated long before the missile arrives near enough to destroy the target and is maintained in only a partially disabled state by the AGC system throughout a substantial portion of the flight of the missile. This allows the warhead to be vulnerable to premature detonation for a comparatively long time.
The magnitude of AGC potential is not the same for jamming signals bearing different types of modulation, for the same average jammer power. Consequently, the degree of passive channel desensitization is different for various types of jamming signal modulation. This has resulted in premature detonation of the warhead, that is, detonation long before the missile had arrived at its destination. Activation of the passive channel is dependent to some degree upon the type of modulation impressed upon the jamming signal. For some types of modulation, the passive channel is never activated, but the active channel has been desensitized sufficiently by the jamming signal so that it also becomes ineffective in initiating warhead detonation, and the target is not destroyed.
For some types of jamming signal modulation, premature detonation occurs because the AGC potential is unable to follow the jamming signal amplitude changes, and consequently the passive channel is inadequately desensitized. The successful operation of the passive system is dependent upon the absolute gains of the active-passive and guard antennas. The absolute gains of the two antennas are not the same at different rotational angles of the missile. At some rotational angles, the two antenna gains are such that the signal delivered to the firing circuit is insufficient to cause warhead detonation.